1. Field of the Invention
The present invention relates to a coating liquid for coating the surface of an image recorded with a pigment ink by an ink jet system, and an ink jet recording method and an ink jet recording apparatus using the coating liquid.
2. Description of the Related Art
With the increase of high-definition images formed by an ink jet system in recent years, ink jet recording apparatus have come to be widely used for public exhibition and trademark printing such as photographs, posters and graphic prints. For this reason, there has been a strong demand for improving fastness properties of resulting images, such as long-term storability.
Inks used in an ink jet recording method are roughly divided into a dye ink containing a dye as a coloring material and a pigment ink containing a pigment as a coloring material. In the dye ink, the dye is dissolved in a molecular state in an aqueous medium. Therefore, a recorded matter formed with the dye ink has such characteristics that transparency is high, and color developability is excellent. On the contrary, the recorded matter formed with the dye ink involves a problem that it is liable to be faded by ultraviolet radiation and active gases in the air. On the other hand, in the pigment ink, the pigment is present in a state dispersed in an aqueous medium. Therefore, a recorded matter formed with the pigment ink is hard to be faded by active gases in the air and ultraviolet radiation, i.e., excellent in resistance to fading by long-term storage.
The advancement of techniques as to pigments in recent years has permitted the recorded matter formed with the pigment inks to satisfy both long-term storability and high color developability comparable to the recorded matter formed with the dye inks. Therefore, spreading of ink jet recording apparatus using pigment inks is progressing mainly in commercial printing uses such as photographs and posters, which are highly required to store recorded matters over a long period of time.
However, recorded matters obtained by an ink jet recording method using pigment inks include the problem that an image formed on a recording medium having glossiness (hereinafter may be referred to as “glossy paper”) for ink jet is easy to be scratched.
A recording medium for ink jet is generally produced by forming an ink receiving layer absorbing an ink on a base material such as paper or a film. In the recording medium, the ink receiving layer contains inorganic fine particles high in absorbency to an aqueous medium, such as silica or alumina in plenty for the purpose of preventing blur of the ink on the recording medium. Since high surface smoothness is required of glossy paper among recording media for ink jet, inorganic fine particles of the submicron order are often used as a material for forming the ink receiving layer. Since a void between the inorganic fine particles, i.e. pore diameter, in the ink receiving layer of such glossy paper is proportional to the particle size of the inorganic fine particles, the inorganic fine particles of the submicron order are used.
On the other hand, a pigment in a pigment ink is dispersed as particles having a diameter of about 100 nm (nanometers). When the particle size of the pigment is more than the pore diameter of the ink receiving layer, the pigment cannot penetrate into the interior of the ink receiving layer and stays on the surface of the ink receiving layer as if the pigment was sifted.
In the glossy paper, the voids in the ink receiving layer are generally often smaller than the particle size of the pigment, so that an image formed on the glossy paper with the pigment ink has such a structure that a pigment layer is formed on the surface of the ink receiving layer. External force at the time of scratching an image is directly applied to the pigment layer forming the image. As a result, the image formed with the pigment ink is easy to be scratched and in some cases, markedly causes such a problem of scratch resistance that the pigment layer is peeled off.
In order to solve such a problem as described above, the following proposals have been made. For example, there is a proposal as to a film-laminating system in which an image is coated with a film (see Japanese Patent Application Laid-Open No. 2000-153677). There are also proposals as to a liquid-laminating system in which the surface of an image is coated with a liquid containing a resin (see Japanese Patent Application Laid-Open Nos. 2004-001446, 2004-099766, 2004-243625, 2005-047053 and 2005-081754). There is further a proposal as to a post treatment system in which a recording medium having an ink receiving layer containing thermoplastic resin particles is used, and the recording medium is heated after conducting recording, whereby a pigment layer is bonded to the ink receiving layer (see Japanese Patent Application Laid-Open No. 2003-170650).
However, it has been known that the above-described prior arts cause the following problems. First, in the film-laminating system, the surface of the image is coated with a resin film having high film strength, thereby achieving a certain scratch resistance. However, such a system involves a problem that the feel of the paper (recording medium) itself is impaired because the surface of the image is coated with the film. There is also a problem that cost becomes high because a laminating treatment is conducted by an off-line system using an apparatus separated from a recording apparatus after forming an image.
On the other hand, there is a problem described below when the liquid-laminating system is adopted. Japanese Patent Application Laid-Open No. 2004-001446 describes that an image-protecting liquid containing a lubricant (mainly, a polyethylene-containing material) is applied to form a protecting layer having a thickness of 1 to 20 μm (micrometers), thereby improving the scratch resistance of an image. However, when a coating film having a thickness of less than 1 μm, by which the feel of the paper can be retained, is formed with this protecting liquid, it is difficult to achieve sufficient scratch resistance, and it is also difficult to prevent a fine scratch caused by contact of a nail. Moreover the coefficient of kinetic friction of the coating layer formed with the protecting liquid cannot be sufficiently lowered by the above-described lubricant.
In the liquid-laminating system, a laminating treatment is conducted by an on-line system using the same recording apparatus just after forming an image. In order to achieve sufficient scratch resistance with materials comprising a conventional liquid for lamination, however, it is necessary that the film has a thickness of several micrometers. Such a thick film involves a problem that the feel of a recorded matter is impaired. For example, Japanese Patent Application Laid-Open No. 2005-081754 describes that a film having a thickness of 1 μm or less is formed. However, still higher scratch resistance is required when taking the circumstances where the recorded matter is actually used into consideration.
The post treatment system described in Japanese Patent Application Laid-Open No. 2003-170650 involves, in addition to need of using a special recording medium, a problem that the apparatus becomes large because a heat treatment is conducted.